1. Field of the Invention
The present invention relates to a process for producing 2,6-dimethylnaphthalene which is useful as a starting raw material for 2,6-naphthalene dicarboxylic acid. 2,6-Naphthalene dicarboxylic acid is industrially important as a starting raw material for polyethylene naphthalate which is employed for producing high-performance polyester in the form of fiber or film having excellent tensile strength and heat resistance.
2. Description of the Related Art
2,6-Dimethylnaphthalene (hereinafter dimethylnaphthalene is sometimes abbreviated to "DMN") is employed as a starting raw material for 2,6-naphthalene dicarboxylic acid and is called upon to have a high purity.
DMN has 10 isomers according to the positions of two methyl groups. It is necessary, however, to produce 2,6-DMN substantially free from an isomer other than 2,6-DMN on a large scale at a low cost. It is well known that in the case of isomerizing DMN, isomerization between adjacent .beta.-positions and isomerization of methyl-migration from one ring to another are unlikely to take place as compared with that between .alpha.-position and .beta.-position. Specifically, the above-mentioned 10 DMN isomers are classified into four groups, namely A to D groups as undermentioned with regard to isomerization, and isomerization among different groups is unlikely to take place as compared with that in the same group.
Group A - - - 1,5-DMN; 1,6-DMN; and 2,6-DMN PA1 Group B - - - 1,8-DMN; 1,7-DMN; and 2,7-DMN PA1 Group C - - - 1,4-DMN; 1,3-DMN; and 2,3-DMN PA1 Group D - - - 1,2-DMN
As the process for producing 2,6-DMN, there are available a process in which naphthalene or methylnaphthalene is methylated, succeedingly isomerized and separated, an isolation process from tar fraction or petroleum fraction and the like. However, the fraction or the reaction product according to any of the above-mentioned processes contains almost all the isomers in the aforesaid four groups, thereby necessitating the isomerization among different groups to take place for the purpose of efficiently producing 2,6-DMN by the isomerization of such isomers. As the method of isomerization among different groups, there is disclosed, for example in Japanese Patent Application Laid-Open No. 88433/1984, a method in which is employed a zeolite having a ten-membered oxygen ring at the inlet of the major cavity. However, the above-disclosed method gives rise to a number of side reactions such as disproportionation in addition to the isomerization because of an elevated temperature required for the reaction, thus lowering a yield of useful 2,6-DMN. That is to say, a process for producing 2,6-DMN by isomerizing a mixture of DMN isomers suffers from the disadvantage that a number of DMN isomers contained that are unlikely to be converted to 2,6-DMN because of their belonging to the group other than the group of 2,6-DMN, lower the yield of 2,6-DMN, necessitate steps of separation from mixed isomers and thereby make the process inefficient as the industrial process for the production of 2,6-DMN.
On the other hand, Japanese Patent Application Laid-Open Nos. 134634/1974, 89353/1975 and 76852/1973 disclose a process for producing o-tolylpentene-2 in high yield from o-xylene and butadiene; a process for producing 1,5-dimethyltetralin by cyclizing o-tolylpentene-2; and a process for producing 1,5-DMN in high yield and in high selectivity by dehydrogenating 1,5-dimethyltetralin, respectively. In the case where 1,5-DMN thus obtained is used as the starting material for the production of 2,6-DMN, it is advantageous in that 2,6-DMN can be produced without difficult isomerization among different isomer groups, since both 1,5-DMN and 2,6-DMN belong the same group with regard to isomerization.
There are proposed a number of processes for producing 2,6-DMN by isomerizing 1,5-DMN, for example, Japanese Patent Publication No. 50622/1972 discloses a method of isomerization in gaseous phase by the use of a silica-alumina catalyst; Japanese Patent Publication No. 4008/1983 discloses a method of isomerization in liquid phase by the use of mordenite containing chromium or the like as a catalyst, and U.S. Pat. No. 4962260 discloses a method of isomerization by the use of .beta.-zeolite or Y-type zeolite.
Nevertheless, the method in Japanese Patent Publication No. 50622/1972, although high in terms of the concentration of 2,6-DMN contained in the reaction liquid, has deficiencies in that considerable amounts of 2,7-DMN and 1,7-DMN are produced which are different in isomerization group from 2,6-DMN and further, considerable amounts of monomethylnaphthalene and trimethylnaphthalene are produced by disproportionation. The method in Japanese Patent Publication No. 4008/1983, although low in the production of 2,7-DMN and in the side reaction such as disproportionation, suffers the defect that the efficiency of isomerization into 2,6-DMN is limited. In addition, the method in U.S. Pat. No. 4,962,260, although low in the side reaction such as disproportionation, suffers the disadvantages that the production of isomers belonging to different isomerization groups such as 2,7-DMN group are high and further the efficiency of isomerization into 2,6-DMN is insufficient.
That is to say, it is tile present situation that any of the prior arts can not attain a high efficiency of isomerization into the objective 2,6-DMN, while suppressing the isomerization into a group different from 2,6-DMN group as well as the side reaction such as disproportionation.
The production of 2,7-DMN not only decreases the yield of the objective 2,6-DMN in the isomerization step, but also brings about the loss of 2,6-DMN and lowers the purity thereof in the step of crystallization/separation after the isomerization step because of the formation of the binary eutectic-mixture of 2,7- and 2,6-isomers and the ternary eutectic-mixture of 2,7-, 2,6- and 1,5-isomers. Moreover, a side reaction such as disproportionation, as might be expected, leads to a decrease in the yield of 2,6-DMN.
Under such circumstances, intensive research and investigation were made by the present inventors on the method of suppressing the formation of DMN belonging to a different group such as the 2,7-DMN group as well as the occurrence of side reactions such as disproportionation in order to develop a method for attaining a high efficiency of isomerization into 2,6-DMN from 1,5-DMN as the starting material. The present invention has been accomplished as the result of such research and investigation.